Particulate agents for dry heat application

ABSTRACT

Particulate agents of this invention typically consist of a hydrophobic, microwave transparent liquid contained in a microwave responsive substrate. Liquid and substrate are selected to be substantially free of attendant moisture. The liquid is readily absorbed into its substrate and retained therein by capillary forces. Aggregates of such particulate agents preheated by microwave energy contain stored dry heat which is transferrable to load objects. The particulate agents may be formulated to provide a desirable fragrance along with the delivery of heat.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention concerns materials which can serve as intermediate agentsfor transfer of heat from an energy source to a load object. Morespecifically, it relates to particulate compositions of matter which canbe used effectively for heating or cooling applications.

In a typical heating application, agent B receives heat from source Aand delivers its heat to load object C. The same applies, in principle,to a cooling application. In that case, agent B receives cold from(gives up its heat to) source A and gives up its cold to (receives heatfrom) load object C. In other words, one may consider the load object ofcold as a source of heat and the source of cold as the load object ofheat. Thus, intermediate agents play a similar role in heating andcooling.

2. Description of the Prior Art

Examples of common heating and cooling applications come to mind quitereadily. Electric corn poppers or hair dryers use air as theintermediate heating agent. Pressing irons or frying pans use metal asthe intermediate heating agents. Hot oil is used as an intermediateheating agent in deep frying and water is used as the primaryintermediate heating agent in most cooking applications. Turning to acommon example of cooling, air is the primary cooling intermediate in arefrigerator. All of the above are instances of dynamic, steady-stateheat transfer, where the intermediate agents receive and discharge heatconcurrently, either intermittently or continuously. Under theseconditions, the agents' ability to transfer heat is of primaryimportance.

In other situations, the process of heat transfer is sequential ratherthan concurrent. In such cases, an intermediate heating (cooling) agentreceives heat (cold) for later use and then delivers it to a loadobject, likely at another location away from the original source.Sequential heating is exemplified by preheated water in a jacketed babyfeeding dish or in a hot water bottle. A freezer pack is an example ofstored cold. It is clear that in sequential heating/cooling applicationsthe intermediate agents' ability to store heat is an importantattribute. Measures of capacity to store heat for materials which do notundergo a change of state relate to specific heat per unit weight and,in combination with density, specific heat per unit volume. Changes ofstate will be considered later.

One other attribute worthy of mention is fluidity; i.e. the agents'ability to flow readily and surround the load object, thereby deliveringheat to more than one surface. It can, and often does, play an importantrole in concurrent as well as sequential schemes of heat transfer, aswill become more evident in the discussion which follows.

Against this background, one can assess the ability of variousmaterials, in their respective physical states, to act as intermediateheat transfer agents.

GASES

Air and combustion gases are readily available, and their fluidity isclearly an advantage. They are well-suited for heating applications, butrelatively slow and energy wasteful, unless the rate of heat transfer toload objects is enhanced by convection. The low specific heat and lowdensity of gases limit their usefulness as agents for storage of heat,except where massive volumes of gas can effectively be employed. Thesame generally applies to cooling.

SOLIDS

Metals are effective agents for transfer of heat by virtue of their highconductivity. However, low specific heat (approx. 0.1 cal/g° C.) andhigh density make them relatively poor agents for storage of heat,except when substantial mass can be brought into play. Inorganic,mineral-type solids are poor thermal conductors, largely unsuitable forheat transfer. Their moderate specific heat (0.2-0.3 cal/g° C.) is notsufficient for storage of heat and cold. Moreover, lacking fluidity,their mode of heat transfer is characteristically unidirectional andmost effective via immediate contact.

LIQUIDS

Because of their physical attributes, liquids are uniquely suited formost heating and cooling applications. They possess fluidity and abilityto transmit heat by conduction as well as convection. Moreover, theirmedium density (0.8-1.2 g/cc) and high specific heat (0.5-1.0 cal/g °C.) make them ideal for storage of heat in sequential processes. Acommon liquid such as water can also exist in other states, therebyextending its effective range of operating temperatures. Water can existas steam for heating applications, with the full benefit of fluidity. Italso can exist as ice for cooling applications, albeit at the expense offluidity. Changes of state enhance the ability of water to store andcarry heat by virtue of the latent heat of condensing steam (forheating) and melting ice (for cooling), over and above what is availablevia sensible heat solely in the liquid state.

Focusing our attention on sequential heating scenarios; i.e. thoseinvolving transfer of stored heat from a source to a load object,liquids would clearly seem to be the preferred choice. However, liquidsare not the panacea for all heating/cooling applications. Many commonliquids, water included, cannot serve as permanent and reusableintermediate agents, because they are volatile, subject to loss byevaporation and therefore in need of frequent replacement. Volatileliquids also risk pressure build-up when heated in hermetically sealedcontainers. Non-volatile liquids are, of course, safer. However,volatile or not, liquids must be effectively contained, secure againstleakage as a result of physical or thermal damage to materials whichcontain them. In any case, liquids are not suitable agents for "dry"heat applications; i.e. where direct contact with such liquids is to beavoided.

Particulate heating/cooling agents, described in U.S. Pat. Nos.4,937,412 and 5,314,005, combine liquids with solids. The resultingcompositions possess physical properties which are intermediate betweenthose of pure solids and those of pure liquids, but with fewer of thedrawbacks aforementioned for either. The particulates in question employporous substrates as carriers for payloads consisting primarily ofnon-volatile liquids. The payloads are intended to increase the specificheat or heat storing capacity of the particles. They also provide themeans for preheating the particles in a microwave oven, since mostcommonly available substrates are relatively microwave transparent. Toachieve that, the liquid must be microwave responsive.

The particulate compositions made according to the state of the art thenexisting were considered to be uniquely suited for healthcareapplications. Heating/cooling pads comprising such particulate matter infabric bags were claimed to have many advantages, among them:

a. Ability to deliver dry heat or moist heat, by specific choices of theliquid component.

b. Automatic recovery of moist heat capability from ambient air; i.e.without need for deliberate resupply of water to replenish moisturegiven up.

c. Fast preheat and reheat by microwave energy rather than conventionalmethods.

d. Repeated usage with minimal handling and preparation.

e. Dual functionality in one product; heat or cold.

f. Benefits from the desirable properties of liquids, such as higherspecific heat, without risk of leakage or drippy mess.

g. Good draping properties even at low temperatures; i.e., neverfreezable to a solid mass.

h. Safer, gradual delivery of heat or cold, characteristic of heattransfer through beds of solids.

Substrates employed by the prior art for particulate agents, typicallycatalyst carriers made of activated alumina and the like, are wellsuited for moist heat applications. They are highly adsorptive tomoisture and readily compatible with hydrophilic liquid payloads. In asense, they may be considered as hydrophilic themselves. That gives themthe facility to deliver moisture, along with heat, and reabsorb moisturefrom ambient sources upon cooldown.

Unfortunately, difficulties arise when, for dry heat applications, it isnecessary to load the same substrates with hydrophobic payloads.Examples of the latter include acetylated monoglycerides,monoglycerides, edible fats and oils, paraffins, paraffin oils andmineral oils. Commonly available substrates are slow to absorb suchpayloads, because the latter must work against ever-present moisture aswell as the properties of the substrate which are inherentlyhydrophilic. In some cases, moisture practically blocks the absorptionof hydrophobic payloads. In others, rising levels of moisture actuallycause the partial expulsion of a payload after it is fully absorbed intoa dry substrate. In yet another instance, the presence of moistureaccelerates the hydrolysis of acetylated monoglycerides, to form freeacetic acid. For any and all of the reasons above, it is advisable topainstakingly shield such "dry heat" particulates from ambient moistureto minimize the presence of moisture in the particles, therebyprecluding its delivery with the application of heat.

Other problems arise with the formulation of dry heat particulates, asdirect extensions of moist heat applications. Hydrophilic payloads arehighly microwave responsive, even more so in combination with water.Particulate agents containing such payloads depend on that microwaveresponsiveness for their preheating, since most commonly availablesubstrates are relatively microwave transparent. Microwave responsivepayloads are so dominant in that regard that the microwaveresponsiveness of the substrate, if any exists, contributes little tothe microwave susceptibility of the particle as a whole.

The aforementioned scenario changes markedly, however, for dry heatapplications. As payloads change from hydrophilic to hydrophobicproperties they also tend to lose their microwave responsiveness. In theextreme, paraffins, paraffin oils or mineral oils are examples ofpayloads which are virtually microwave transparent. Under thosecircumstances the microwave responsiveness of the substrate becomesall-important. Commonly available substrates carrying such payloads areessentially microwave transparent. Therefore, microwave responsivenesscan only be restored by coupling microwave transparent payloads withmicrowave responsive substrates.

In summary then, it is clear that particulate agents intended for dryheat application require substrates which are:

1. Minimally attractive to moisture

2. Readily absorptive to hydrophobic payloads

3. Microwave responsive, for accommodating payloads which are inherentlymicrowave transparent

Further development of this technology has recently addressed each andevery one of the above requirements. Accordingly, the object of thisinvention is to provide the practitioner with alternate particulatecompositions which extend well beyond the prior art. More specifically,a further object of the invention is to suggest particulates withcompositions better suited for dry heat applications.

SUMMARY OF THE INVENTION

The present invention identifies particulate compositions of matterwhich are better suited for the delivery of dry heat. The particlesconsist of non-volatile, microwave transparent, hydrophobic substanceswhich are contained in macroporous, microwave responsive, solidsubstrates. The substances are readily absorbed into their carriers andheld therein by capillary forces. They are carefully selected to becompatible with each other and substantially free of attendant moisture.Aggregrates of such particles, suitably contained, may be preheated bymicrowave energy. Heat stored therein is available for delivery by closeproximity of the particles to load objects. The delivery of heat may beaccompanied by a desirable fragrance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated in the accompanying drawings ofwhich:

FIG. 1 is a sectional view of a particle with uniform porositycontaining absorbed liquid;

FIG. 2 is a sectional view of a particle with a well defined, enlargedcavity similarly containing absorbed liquid; and

FIG. 3 is a graphic presentation, comparing the thermal performance ofvarious particulate compositions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention recognizes the advantages which have been cited by theprior art for the use of particulate heating/cooling agents. It goesbeyond that prior art by addressing and resolving problem areas whichhave existed relative to the delivery of dry heat. More specifically, itdoes so by deliberately and systematically minimizing the presence anddelivery of moisture in the course of heat application.

The particulate agents of this invention employ hydrophobic substancesas payloads for heat storage enhancement. Because such substances arenot apt to attract or contain moisture, they inherently tend to deliverdry heat only. Among the substances available for this purpose, roughlyin order of decreasing microwave susceptibility, are acetylatedmonoglycerides, monoglycerides, edible fats and oil, paraffins, paraffinoils and mineral oils. All of the above are non-toxic and most areeither pharmaceutical-grade or food-grade substances. Some, such asmonoglycerides and paraffins, are solid at room temperature. Therefore,they must be melted for absorption into a carrier substrate. Of theoptions remaining, minerals oils or paraffin oils are clearly preferredfor this purpose by virtue of their remarkable chemical stability, whileother are subject to decomposition and oxidation under repeated heatcycling.

Because the preferred payload, mineral oil, is substantially microwavetransparent, the choice of substrate becomes an important matter. Notonly must the substrate be low in moisture and readily absorbent to ahydrophobic liquid such as mineral oil, it must also be microwavesusceptible. Otherwise the particles would not be responsive to andheatable by microwave energy.

Conventional substrates such as catalyst carrier aluminas are notsuitable for this purpose for reasons already enumerated. For laterreferfence, let us denote them as substrate A. What we need is asubstrate which is readily absorbent to hydrophobic liquids such asmineral oils and at the same time minimally attractive to moisture. Thelatter will assure that the substrate per se does not contributemoisture to the heating function. It so happens that intensive thermaltreatment of Type A substrate can in fact produce a substrate with theproperties desired. To make the distinction clear, we shall refer to itas substrate B.

Aluminum Corporation of America makers of Type A substrates have, at ourbehest, produced test quantities of Type B substrates by a thermaltreatment. We have used such substrates extensively in the developmentof this invention. Information provided by ALCOA indicates that Type Bsubstrates typically differs from Type A in the nature of their porousstructure and related properties, as follows:

    ______________________________________                                                          Type A  Type B                                              ______________________________________                                        Particle size, Tyler                                                                              7-14 mesh 7-14 mesh                                       Bulk density lbs/ft.sup.3                                                                         38.5      35.0                                            Particle density lbs/ft.sup.3                                                                     64.2      58.3                                            Specific surface m.sup.2 /gm                                                                      330       107                                             Mesoporosity, cc/gm 0.18      0.36                                            30-750 angstrom pores                                                         Macroporosity, cc/gm                                                                              0.33      0.45                                            750 angstrom pores                                                            Total porosity, cc/gm                                                                             0.51      0.81                                            Avg. Pore Diameter, angstrom                                                                      62        303                                             (calculated)                                                                  Static Moisture Sorption, %                                                                       19.5      5.0                                             at 58% RH                                                                     ______________________________________                                    

It is evident that the properties of Type B substrate, notably its extraporosity and its diminished affinity for moisture, are credible measuresof the attributes desired for this invention. Substrates having a totalporosity greater than 0.6 cc/gram and specific surface area smaller than200 m² /gram are uniquely suited to the hydrophobic payloads of thisinvention.

Turning now to the drawings, FIG. 1 shows a substrate particle 10 withuniform or continuous porosity. The particle holds a diffusely absorbedliquid payload 11 by capillary action. It also shows some unfilledabsorption capacity which, for the purpose of illustration, is depictedas a liquid deficient outer shell 12. FIG. 2 shows a similar particle20, with variable or discontinuous porosity. Part of payload 21 is showncontained in that particle inside an enlarged, chamber-like cavity 22.Generically speaking, cavity 22 also represents the sum total of manyrandom-sized vacuoles which are typically present in increasinglymacroporous structures. Headspace 23, not filled with liquid, is onceagain indicative of deliberate liquid insufficiency. As a general rule,it is prudent to restrict the amount of liquid relative to substrate, sothat differences in thermal expansion between the materials do notresult in the expulsion of liquid out of its substrate with changes intemperature.

To accommodate hydrophobic payloads which are relatively orsubstantially microwave transparent, it is clearly desirable to make theideal substrate microwave responsive. Solids owe their microwaveproperties to their chemical and/or ionic structure. However, to thebest of our knowledge, microwave susceptibility is not normally a matterof concern in the manufacture of catalyst carriers. The incidence ofsuch properties does not seem to be either predictable or controllableat present.

In searching for microwave responsive substrates it is best to simulatethe conditions under which they are to be used. Briefly, this consistsof loading a fixed weight of substrate with a fixed weight of payload,subjecting the mass to a dose of microwave energy, and then noting itstemperature peak and subsequent cooldown. All this is done underreplicable conditions, with the only variable being the substratetested.

By means of such testing we have discovered two random substrates whichare microwave responsive. One is Type A and the other Type B, both madeby ALCOA. Thus, while most substrates are relatively microwavetransparent (denotable as MWT), some substrates happen to be microwaveresponsive (denotable as MWR). Of the four options possible, i.e.substrates A/MWT, A/MWR, B/MWT and B/MWR, we clearly prefer the last.Substrate B/MWR possesses all of the attributes desired for thisinvention. It has high absorbency for hydrophobic payloads, low affinityfor moisture and, most importantly, microwave responsiveness. This andother aspects of the invention will become more evident from a series ofexperiment performed in the course of its development.

Methods used in searching for the desired substrates can also be used todemonstrate the essence of this invention. A series of such experimentscompares the performance of various substrates under well-controlled,replicable conditions. The substrates under comparison, typicallyspherical particles about 1/16- 1/8" in diameter, are all derived fromaluminum-based catalyst carriers. One hundred grams of each substratewere loaded with sixty grams of a hydrophobic payload. The particleswere then transferred to thermally insulated cups and microwaved,covered, for a prescribed period at a fixed power level. Immediatelyafter microwaving, a thermometer probe was inserted through the coverinto the core of the particulate aggregate. Temperature was thenobserved and recorded until the core reached about 140 ° F., andend-point chosen arbitrarily for purposes of comparison.

The results of these tests are shown graphically in FIG. 3 with thefollowing combinations of materials.

    ______________________________________                                        Combination                                                                             Substrate Payload       MW Time                                     ______________________________________                                        I         B/MWT     Monoglyceride 1.5 min.                                    (Ia)      (A/MWR)   (Monoglyceride)                                                                             (1.5 min.)                                  II        B/MWT     Mineral Oil   3.0 min.                                    III       A/MWR     Mineral Oil   2.0 min.                                    IV        B/MWR     Mineral Oil   2.5 min.                                    ______________________________________                                    

As expected, the above hydrophobic payloads were extremely slow toabsorb into Type A substrates even with the latter carefully predriedand substantially free of adsorbed moisture. Monoglycerides, with amelting point of 158 ° F., had to be melted prior to their absorptioninto the substrates. Results obtained under combination Ia, not showngraphically but referred to in the table within parentheses, were notdiscernibly different from those of Combination I. That clearlydemonstrates that for moderately microwave responsive payloads such asmonoglycerides it makes no difference whether the substrate is microwaveresponsive or not.

Of the results shown graphically, it is important to note the following:

1. Microwave transparent substrate, type B/MWT, depends on the moderatemicrowave susceptibility of the payload to perform as it does in graphI. Note, however, what happens in graph II when the moderatelyresponsive payload, monoglyceride, is replaced by a transparent payloadsuch as mineral oil. That combination is practically ineffective evenafter doubling microwave time.

2. Microwave responsiveness is restored almost fully by matching thetransparent payload, mineral oil, with microwave responsive substratessuch as A/MWR or B/MWR. Note that graphs III and IV closely approximatethe performance of graph I, albeit with additional microwave time. Moreresponsive substrates, when available, will probably require less time.

The particulate agents of this invention are well suited for therapeuticdry heat applications. A limb, body part, or extremity may, for example,be immersed directly into an aggregate of preheated particles which areheld in an open container. The particles may also be contained in aflexible bag made of fabric, much like a bean bag, and then be used inthat fashion. The bag may in fact be designed, sized and shaped toreadily fit around specific parts of the body.

The same technology may also be used to add a desirable fragrance alongwith the application of heat. Heat applications to children may beaccompanied by a fragrance suggestive of baby oil, baby powder and thelike. Heat applications to adults may, similarly, be enhanced byfragrances suggestive of cleanliness, wholesomeness, medication oranything that will make for a more pleasurable experience. To achievesuch objectives, an oil-soluble fragrance component may simply beincluded in the hydrophobic payload of the particles. The fragrancecomponent may be present uniformly in each and every particle, or belimited to a select proportion of the particles which is evenlydistributed in an otherwise non-fragrant aggregate.

Primary consideration has thus far been given to applications ofsequential heating; i.e. those entailing heat generation in themicrowave and later delivery of stored heat to a load object outside themicrowave. However, with the property of fluidity, particulate solids ofthis invention may be used as sources of heat inside the microwave. Anyobject which is inherently microwave transparent can be immersed in abed of such solids and be heated by them while the combination isundergoing microwaving.

The foregoing description is for the purpose of teaching the person ofordinary skill in the art how to practice the present invention. It isnot intended to detail all of those obvious variations and alternativeswhich will become apparent to the skilled practioner upon reading thedescription. It is intended, however, that all such variations andalternatives be included within the scope of the present invention whichis defined by the following claims.

I claim:
 1. Particulate agents for dry heat application comprising:a) asolid substrate derived via intensive thermal treatment an alumina-basedadsorbent and catalyst carrier which is relatively microwave transparentand microporous, with a total porosity greater than 0.6 cc/gram and aspecific surface area smaller than 200 m² /gram; and b) a hydrophobicsubstance, absorbed in said substrate, which is moderately microwavetransparent, said substance comprising at least one major, non-volatilecomponent.
 2. The particulate agents of claim 1 wherein the hydrophobicsubstance includes a component selected from the group consisting ofmonoglycerides and other fatty acid esters of polyhydric alcohols. 3.Particulate agents for dry heat application comprising:a) a solidsubstrate derived via intensive thermal treatment an alumina-basedadsorbent and catalyst carrier which is relatively microwave transparentand microporous, with a total porosity greater than 0.6 cc/gram and aspecific surface area smaller than 200 m² /gram; and b) a hydrophobicsubstance, absorbed in said substrate, which is substantially microwavetransparent, said substance comprising at least one major, non-volatilecomponent.
 4. The particulate agents of claim 3 wherein the hydrophobicsubstance includes a component selected from the group consisting ofedible fats and oils.
 5. The particulate agents of claim 3 wherein thehydrophobic substance includes a component selected from the groupconsisting of paraffins, paraffin oils and mineral oils.
 6. Theparticulate agents of claim 3 wherein the hydrophobic substance furthercomprises a volatile fragrant additive for delivery of a desirablefragrance along with the delivery of heat.